Cryogenic pumps are pumps for use in pumping cryogenic fluids, i.e. fluids at cryogenic temperatures, including for example liquefied fuel gases such as liquefied natural gas (LNG). In this specification, cryogenic temperatures are taken to be temperatures below −100° C., typically below −150° C.
Cryogenic pumps may be used for example to transfer cryogenic fluids during production and transportation or to deliver a cryogenic fuel to an internal combustion engine such as a reciprocating dual fuel compression ignition engine. Such engines operate on a mixture of a gaseous fuel, such as natural gas, and a petroleum distillate fuel, such as diesel, but require high gaseous fuel pressures in order to achieve high power density at high gaseous fuel-to-petroleum distillate substitution rates.
In such applications it is known to pump a cryogenic fluid by driving a cryogenic pumping element, e.g. a piston reciprocating in a cylinder, via a pushrod assembly powered by an actuator such as a hydraulically driven piston. The elongate pushrod helps to thermally separate the cryogenic pumping element at the cold end of the assembly and the hydraulic actuator at the (relatively) warm end, so that the hydraulic drive components are able to operate at ambient temperature to provide the required fuel pressure.
For example, US 2014/334947 discloses a reciprocating pump assembly suitable for pumping LNG, comprising a drive rod and a pump rod with crowned ends. The drive rod and pump rod are releasably coupled together in axial relation by a connector which accommodates slight misalignment.
If a hydraulic actuator is used to drive the cryogenic pumping element then an effective sealing system is required to ensure that the hydraulic fluid does not contaminate the LNG or other cryogenic fluid, or vice-versa. However, this can be difficult in cryogenic service conditions, particularly in view of the different coefficients of thermal expansion of cryogenic sliding seal materials such as polytetrafluoroethylene (PTFE) and ultra high molecular weight polyethylene (UHMWPE) relative to that of stainless steels and other materials commonly used for pistons and other cryogenic pump components.
It is known to install an annular seal between a shaft and its housing by stretching and sliding the seal axially along the shaft. However, it can be difficult to install and remove such seals while ensuring adequate energisation in the use position after the seal relaxes into a groove in the shaft. In cyrogenic applications, leakage may result from thermal contraction of the shaft.
It is also possible to exploit differential thermal expansion to provide sealing in a cryogenic system which operates at a constant temperature. For example, U.S. Pat. No. 6,547,250 discloses a cryogenic shaft seal assembly comprising a spring energised seal supported by a static metal seal shrunk onto the shaft.
However, in many cryogenic systems the service temperature is not constant. For example, in mobile applications large temperature changes occur due to normal service and refuelling cycles. When the pump is not in use, the warm end of the assembly will lose heat by conduction to the cold end at a rate depending inter alia on the ambient temperature at the warm end which also fluctuates with the operating environment of the vehicle.
The use of an exposed pushrod to couple an actuator to a pumping element arranged at a distance from the actuator avoids cross-contamination between the hydraulic and cryogenic fluids due to leakage at the seals. However, it is more difficult to avoid cross-contamination when the pushrod must be enclosed in a housing, for example, in order to provide a more compact assembly to provide fuel to a vehicular engine, since the housing provides a path through which leaking fluids may migrate.
US 2016/0215766 A1 discloses a pump for a cryogenic fluid comprising pistons operated by oil lubricated pushrods. The pushrods have oil seals and vents to relieve pressure from the annular space between the pushrods and the housing to reduce leakage through the oil seals.